docs: def clean_account(self):
          """Ensure this is an income account"""
          account = self.cleaned_data['account']
          if not account:
              return
  
          if account.type != Account.TYPES.income:
              raise ValidationError('Account must be an income account')
  
          try:
              account.housemate
          except Housemate.DoesNotExist:
              pass
          else:
              raise ValidationError('Account already has a housemate')
  
          return account